Sensor with glass seal

ABSTRACT

A sensor with a glass seal between a planar sensing element in a tubular housing and the tubular housing, wherein the glass seal is maintained in radial compression over an entire temperature operating range of the sensor. The compressed seal has improved strength and durability, is self-limiting and is operable over a large temperature range and in an environment subject to jarring and vibrations.

This invention relates to a sensor with a glass seal apparatus andmethod of manufacture.

BACKGROUND OF THE INVENTION

Example glass sealing methods for an oxygen sensor are set forth in U.S.Pat. No. 5,329,806, assigned to the assignee of this invention. Insensors such as in the '806 patent, the purpose of the glass seal is toisolate an end portion of an oxygen sensor from an air reference channelwithin the sensor. The seal prevents the gas being sensed, i.e.,automotive exhaust gas, from leaking into and interfering with the airreference in the channel and the detrimental effect on the measurementoutput of the sensor that would result from such a leak.

In some prior attempts at achieving the glass seal, the glass seal hasbeen implemented as both a structural and sealing element. Such sealshave been prone to failures, including structural cracks allowingleakage into the air reference channel, failure of the glass to adhereto the outer housing of the sensor, and failure to maintain an effectiveseal over a wide operating temperature range such as required by anexhaust gas oxygen sensor.

Additionally, there is a chance that the sealing glass will seetemperatures in excess of the glass transition temperature, in whichcase the region between the glass sensor and the shell of the sensor isin tension, impairing the structural integrity of the seal. Leaving theglass in tensile stress can lead to cracking in the seal, which allowsinfiltration of the sensed gas, i.e., automotive exhaust, into the airreference channel.

SUMMARY OF THE PRESENT INVENTION

It is an object of this invention to provide a sensor with a glass sealin accordance with claim 1.

Advantageously, this invention provides a sensor with a glass sealcapable of sealing a flat plate sensing element within a circular sensorhousing.

Advantageously, this invention provides a new sensor and glass seal thatprevents exhaust and other external elements from contaminating the airreference channel in the sensor.

Advantageously, this invention provides a sensor with a glass seal thatcan be used to seal a planar sensing element in either a ceramic or ametal sensor housing.

Advantageously, this invention provides a sensor with glass seal inwhich the glass seal coefficient of thermal expansion, transitiontemperature and melting point are all controlled to maintain acompressive seal within the housing of the sensor allowing the seal tomaintain integrity at maximum exhaust sensor service temperatures.Accordingly, this invention provides a sensor with glass seal in whichthe glass seal is in compression during the entire operating range ofthe sensor, maintaining a glass to metal or ceramic seal.

Advantageously, according to this invention, a glass to metal seal ismaintained by providing a seal with a coefficient of thermal expansionin a range equal to the coefficient of thermal expansion of the sensingelement and less than the coefficient of thermal expansion of the metalsensor shell wherein the metal sensor shell holds the glass incompression, allowing the glass seal to maintain its maximum strengthover the entire operating range of the sensor. Advantageously, bymaintaining the glass seal in compression, the seal according to thisinvention can withstand conditions of shock and vibration in an engineenvironment and has improved mechanical integrity.

Advantageously, an example sensor with glass seal according to thisinvention is achieved with a glass seal having a flat circular diskportion, including a centrally located rectangular opening in which aplanar sensing element is located. At an outer radial periphery of theflat disk portion, the seal forms a circular cylindrical wall extendingaxially away from the flat circular disk portion in first and seconddirections opposite to each other. The seal is placed within the sensorhousing wherein the circular cylindrical wall engages the innercylindrical wall of the sensor housing and wherein the rectangularopening engages the planar sensing element. The seal is located betweenfirst and second insulators that act to structurally locate the seal andsensor element. The outer cylindrical surface of the seal bonds with thecircular wall of the sensor housing and the rectangular opening of theseal bonds with the sensing element, wherein the seal seals a first endof the sensing element in a sensing chamber from a second end of thesensing element in an air reference channel in the sensor. In oneexample, the flat circular disk portion has a thickness less than thethickness of the planar sensing element.

Advantageously, then, in one example this invention provides a method ofmanufacturing a sensor with a glass seal, comprising the steps of:placing a glass seal having a circular disk portion with a rectangularopening centrally located therein and an annular cylindrical wallextending first and second axial directions from a periphery of thecircular disk portion into a sensor housing wherein an outer cylindricalsurface of the annular cylindrical wall of the seal is proximate to aninner cylindrical wall of the housing and wherein a sensing elementpasses through the rectangular opening in the circular disk portion ofthe seal; heating the sensor package to a temperature at which the glassof the seal melts, wherein the annular cylindrical wall of the sealmelts and flows to the inner cylindrical wall of the shell and whereinglass around the perimeter of the sensing element flows around thesensing element; and cooling the sensor, wherein said glass cools andmaintains a seal between the sensing element and the outer shell of thesensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the following drawings in which:

FIG. 1 illustrates a first example sensor and glass seal according tothis invention;

FIGS. 2 and 3 illustrate an example glass seal according to thisinvention;

FIG. 4 illustrates a second example sensor with glass seal according tothis invention;

FIG. 5 illustrates example benefits of the sensor with glass sealaccording to this invention;

FIG. 6 illustrates a stress analysis of the sensor with glass sealaccording to this invention;

FIG. 7 illustrates a third example sensor with glass seal according tothis invention;

FIG. 8 illustrates the method according to this invention;

FIG. 9 illustrates the operation of this invention; and

FIG. 10 illustrates another example of this invention.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLE 1

In a first example, a glass seal in a sensor is achieved by reducing thethickness of the seal designated as reference 48 in FIG. 3 of the abovementioned U.S. Pat. No. 5,329,806 so that it is thinner than the planarsensing element or on the order of about one millimeter or less.Referring to FIG. 1, the glass seal 13 according to this invention isprovided with a central opening rectangular in shape to match the shapeof the planar sensing element 21, which extends through the opening. Oneither side of the seal 13, then, are placed two insulators 11 and 15 ofsteatite glass having melting temperatures higher than that of the seal.The sensor is treated in a furnace to melt and flow the glass of theseal to engage the inner cylindrical wall of the sensor housing (outershell) 17 and the outer surface of the planar sensing element 21. Thesensor is then cooled so that the glass hardens, sealing the sensorwithin the housing and providing an air tight seal separating the firstand second ends of the sensor.

The furnace treating of the sensor must be maintained for a timesufficient to allow the glass of the seal 13 to flow out to meet thesensor housing 17. Special care must be taken to maintain pressure onthe seal 13 perpendicular to the seal as pressure provided at an anglecan cause irregular flow of the glass and the possibility of leaving anair gap between the seal and the shell of the sensor.

To prevent the glass from deforming the sensor housing 17 during thecooling step, the yield strength of the sensor housing must be greatenough so that the housing maintains the glass in compression and doesnot yield to the glass. If the housing 17 yields to the seal 13, thehousing 17 becomes deformed around the seal and, during operation athigh temperatures, the housing expands, placing the seal in tension,leading to failure of the sealing bond between the glass and the housingand/or cracks in the seal, resulting in an imperfect seal. A housing 17with appropriate yield strength may be achieved by providing a thickenough wall of the housing 17 and/or by appropriate selection ofmaterial, i.e., 0.018"-0.024" thick SAE 486 stainless steel.

The fitting 19 is mounted around the housing 17 and is used for mountingof the sensor as described below.

EXAMPLE 2

Referring now to FIGS. 2 and 3, a preferred example glass seal accordingto this invention is designated by reference 10 and generally comprisesa circular shaped flat disk portion 12 having centrally located thereinan opening 22 defined by walls 14, 16, 18, and 20 for engaging a planarsensing element having a shape substantially matching that of theopening 22. Preferably the thickness of the flat disk portion 12 is lessthan the thickness of the planar sensing element and, in one example, ison the order of one millimeter. At the radial periphery of the flat diskportion 12, the seal 10 has an annular cylindrical wall 24 extendingaxially away from the flat circular disk portion 12 in both directionsso that the flat circular disk portion 12 is substantially centrallylocated within the cylindrical tube formed by the cylindrical wall 24.In one example, the cylindrical wall 24 has an axial length of 4 mm anda thickness of between 0.5 and 1.0 mm.

The cylindrical wall 24 has a top annular end 26 and a bottom annularend 28, inner annular cylindrical surfaces 34 and 32 and an outercylindrical surface 30. When the sensor is constructed with the seal 10shown, the outer cylindrical surface 30 bonds to a sensor housing andthe inner peripheral walls 14, 16, 18, and 20 of the opening 22 bond toa sensing element to seal a first end of the sensing element in achamber exposed to a gas to be sensed, i.e., vehicle exhaust gas, awayfrom a second end of the sensing element in an air reference channel.

FIG. 4 illustrates an example sensor with glass seal according to thisinvention. The sensor 40 generally comprises an outer housing comprisinglower shell 42 and upper shell 63 formed from a material such as 486steel having a coefficient of thermal expansion in the range of 11×10⁻⁶to 12×10⁻⁶ /° C. Upper shell 63 has a peripheral fold 65 around the lip67 of the lower shell 42, joining the upper and lower shells 63 and 42together. Annular fitting 50 fits around shell 63 above fold 65. Thefitting 50 is adapted to mate with a receptacle for mounting the sensor40 in a gas flow path.

Sensing element 44, constructed in a manner known to those skilled inthe art, such as described in U.S. Pat. No. 5,329,806, assigned to theassignee of this invention, is generally a flat planar sensing elementhaving an end 45 exposed to exhaust gases in chamber 41 for sensing theoxygen content of the exhaust gases therein. A glass support 46 locatesthe sensing element 44 within the axial opening 47 of insulator 48.Insulator 48 is formed, for example, from a steatite type materialhaving the characteristics of low thermal conductivity. During sensoroperation, gases entering chamber 41 may be of temperatures as high as950° to 1000° C. The insulator 48 insulates the glass seal 10 from suchhigh temperatures and the glass seal 10 never sees temperatures higherthan the glass transition temperature, which, in this example, isapproximately 720° C. Typically, the highest temperature that glass seal10 reaches during normal sensor operation is 650° C.

The glass seal 10 is shown located between insulator 48 and insulator56, which is formed of alumina and has a coefficient of thermalexpansion similar to insulator 48. A third insulator 57 is connected byglass 58 above insulator 56. The seal 10 has been heated and bonded toboth the sensing element 44 and the outer shell 42 of the sensor in theregion 54.

More particularly, during the furnace heating of the sensor, the glassseal 10 melts and, through capillary action, flows a small distance inthe axial direction along the sensing element 44 as shown by reference55. When the sensor is cooled, the glass is bonded to the sensingelement 44. Also during the heating process, the cylindrical wall 24 ofthe seal 10 flows into contact with the inner periphery of shell 63 inthe region 54 so that when the sensor is cooled, the glass is bondedthereto. The cylindrical wall 24 provides material to flow and meet theshell 63, eliminating the reliance on capillary action to draw the glassaxially along the periphery of the shell 63 in the region 54 to form asealing bond, which in turn does not cause a build up of tension alongthe shell.

Shell 63 has a coefficient of thermal expansion greater than that of theglass seal 10. For example, the glass seal 10 may comprise a Srborosilicate glass with a coefficient of thermal expansion of 7.8×10⁻⁶/° C. As a result, the shell 63 expands faster than the glass of theseal 10 during the furnace heating and treatment. However, because thecylindrical portion 24 provides sufficient material to flow out to meetthe region 54 of the shell 63, the greater coefficient of thermalexpansion of shell 63 does not adversely affect the sealing capabilityof the glass seal 10. Rather, because the glass flows out to meet theexpanded shell in the region 54, as the sensor cools and the shellcontracts, the glass seal 10 solidifies and then enters into a state ofcompression being compressed by the shell 63 in the region 54. Thisstate of compression strengthens the seal as glass is inherentlystronger under compression. Further, this process eliminates thepossibility of the glass seal 10 undergoing tension during operation ofthe sensor since the glass seal region of the sensor is at an operatingtemperature range below the temperature at which the glass seal melts.Thus, in an exhaust environment where chamber 41 of the sensor istypically exposed to temperatures in the range of -40° to +1000°Celsius, the glass seal 10 region is maintained in compression andtherefore in a state of structural integrity since it never sees atemperature greater than the glass transition temperature.

Additionally, the planar sensing element has a coefficient of thermalexpansion equal to that of the glass, preventing the glass around theplanar sensing element from undergoing tension. An example coefficientof thermal expansion of the planar sensing element is 8×10⁻⁶ /° C.

Referring to FIG. 5, small portions of the outer periphery of thecylindrical wall of the glass seal 10 may, through capillary action,flow axially between insulator 48 and shell 63 or insulator 56 and shell63 and such portions 106, 108, 110, and 112 may end up in tension.However, as shown with reference to FIG. 5, cracks (92 and 94) that maydevelop due to the tension in the capillary flow areas inherently turnwhile propagating to dead end against the insulator 48 or 56 on the sameside from which the crack started. The cracks thus become self limitingand do not translate across the seal 10 to form leak paths for exhaustgas into the air reference chamber. These results have been verified byexperimentation and analysis of sample sensors according to thisinvention.

During the furnace treatment, portions of the glass 96, 98, 100, and 55are drawn axially through capillary action to help seal around sensingelement 44.

Referring again to FIG. 4, the remainder of the sensor includes theterminals 62 and 64 bonded to the sensing element 44 in a known mannerand terminating in male terminal ends 66 and 68, which engage femalereceptacles 70 and 71 of the connector 79. The female receptacles 70 and71 are tubular shaped metal receptacles constructed in a manner known tothose skilled in the art and are crimped (at 74 and 75) to wires 82 and84 for electrical connection in a known manner. The receptacles 70 and71 are retained in the insulator 72, which is sealed by O-ring seal 76to the end 43 of the shell 63.

Connector 79 has an outer shell 78 including a cantilever retainingfeature 81 for maintaining the shell and connector 79 on the end 43 ofthe sensor shell 63. The end of the connector includes a floralelastomer seal 80 for maintaining the wires in place and for sealingagainst water contamination from outside the sensor. The end of theouter shell 78 is folded as shown by reference 83, maintaining theinsulator 72 in place.

Referring to FIG. 6, the example stress analysis shown illustrates thepositive pressure on the glass seal 10 due to the seal structure,including the cylindrical walls 24 according to this invention. Thestructure shown illustrates compressive stress on the glass seal 10 inan example operating temperature of -40° Celsius, which is the lowestexpected operating temperature of the example sensor shown and thetemperature at which the glass seal experiences the greatest amount ofcompressive stress along the shell region, increasing slightly acrossthe seal region toward the element by approximately 30 MPa. The portionsof the seal and insulators designated by references 128 and 120,respectively, have the highest compressive stress, peaking at -93.11MPa. The regions (122, 124, and 126) and (130 and 132) illustrategradually reduced stress regions in the ceramic component in an areajust below the seal region.

The figure illustrates the compressive stress profile of the seal 10under the condition of greatest stress and the structural integrity ofthe seal under such conditions that allow the seal 10 to be an effectivemechanical structure while maintaining the glass to metal seal. Becauseof the high compressive stress level of the glass 10, the glass seal 10can now maintain integrity in the environment of the vehicle enginewhere exposure to vibrations and wide operating temperature ranges iscommon.

EXAMPLE 3

Referring now to FIG. 7, another example sensor with a glass sealaccording to this invention is shown. In the example shown in FIG. 7,the glass seal 214 seals between a tubular ceramic insulator 210 and theplanar sensing element 202 in much the same way that the seal 10 in FIG.4 seals between the metal shell and planar sensor 44.

More particularly, the sensor 201 shown comprises a lower shield 200attached to a steel fitting 208 and provides a chamber for exposure ofexhaust gas to the end 203 of the planar sensing element 202. A glasssupport 204 maintains the sensing element 202 in place with respect tolower insulator 206 comprising a steatite material similar to lowerinsulator 48 in FIG. 4. The glass support 204 also shortens thecantilever of the element with respect to the package. An intermediateglass spacer 212 is provided between lower insulator 206 and middleinsulator 209. The glass seal 214 according to this invention, similarto glass seal 10 described above, is provided between the middleinsulator 209 and the upper insulator 216 as shown. An example materialfor the glass seal 214 is Ba borosilicate having a coefficient ofthermal expansion in the range of 5×10⁻⁶ to 7×10⁻⁶ /° C. The annularcylindrical wall of the seal 214 interfaces with the inner cylindricalwall 215 of the ceramic insulator 210 (the housing). During a furnaceheating process similar to that described above with respect to FIG. 4,the material of the glass from the cylindrical outer wall of the seal214 flows into contact with the ceramic insulator 210. The ceramicinsulator 210 has a coefficient of thermal expansion greater than orequal to that of the glass seal 214. During cooling, the glasssolidifies, bonding with ceramic insulator 210, providing a seal betweenthe glass seal 214 and the ceramic insulator 210.

As with the sensor with the steel shell 63 above with respect to FIG. 4,after the furnace heating, when the sensor has cooled, the seal 214 issolidified in sealing contact with the ceramic insulator 210 and thesensing element 202. The resulting glass seal 214 is in compressionsimilar to the way that seal 10 is in compression in FIG. 4,guaranteeing the integrity of the seal and preventing the flow ofexhaust from the end 203 of the sensing element 202 to the air referencechannel near end 225 of sensing element 202.

The ceramic insulator 210 is mounted within the steel body 208 thatcomprises the fitting for the sensor and a seal is provided between theceramic insulator 210 and the steel body 208 by gasket 213 between theshoulders 250 and 252 of the ceramic insulator 210 and steel body 208,respectively. A disk spring 254 is provided between the upper end of theceramic insulator 210 and the insulating spacer 256, providing pressureon the ceramic insulator 210 maintaining sealing compression of thegasket 213. The insulating spacer 256 is held in place by the crimp 258on the upper end of the steel body 208.

In the wedge opening of the upper insulator 216, terminals 220 and 222are attached to the sensor element 202 in a known manner. Terminals 220and 222 extend upward and form male terminal ends 227 and 229 engagingfemale receptacles 230 and 228 within insulator 231. Spacer 226 andglass 224 are located between insulator 231 and upper insulator 216. Theupper end of the sensor 201 is enclosed in the steel shell 260, crimpedto the outer periphery of the steel body 208, enclosing the terminal endof the sensor. The female receptacles 230 and 228 include crimped ends232 and 234 located within insulator 264 for engaging the wires 236 and238 providing electrical connection to the sensor 201. The wires 236 and238 pass through viton seal 240, which is maintained in place by thecrimped end 242 of the shell 260. Within the upper shell 260, a metalretainer 262 is fit in place, for example by a friction fit, to providea positive locating stop of the insulator 264.

Referring now to FIG. 8, an example method of manufacturing a sensorwith a glass seal according to this invention is shown. At step 302, theglass seal 10 (FIGS. 2-4) is fabricated to include an annular outer bodyand a flat disk portion with a central opening matching the shape of aplanar sensing element 44 (FIG. 4). At step 304, the glass seal isplaced around the planar sensing element so that the planar sensingelement passes through the opening and the combination is placed in acylindrical support (shell 42, FIG. 4) having a coefficient of thermalexpansion greater than that of the glass of the seal (block 304). Thecombination is then heated (block 306) to expand the cylindrical supportat a rate faster than the rate of expansion of the glass seal and tomelt the glass of the glass seal so that the glass flows from theannular outer body of the seal out to the expanded cylindrical supportinner wall and from the periphery of the central opening to the planarsensing element. The combination is then cooled so that the glass sealhardens in a state bonded to the sensing element and to the cylindricalsupport (block 308). The cooling of the combination is continued (block310) so that the glass seal is in compression over the entire operatingrange of the sensor. The result is a sensor with a robust glass sealsealed to both the planar sensing element and the cylindrical support.

FIG. 9 illustrates the relationship between the sensor design,manufacture, and characteristic compression of the seal. The horizontalaxis illustrates temperature, which is increasing to the left, and thevertical axis illustrates stress on the seal. When the sensor is heatedduring manufacture to a temperature T₁ (for example 950° C.) greaterthan the highest operating temperature of the glass seal region of thesensor, the glass seal becomes fluid and glass from the outercylindrical wall of the seal flows out to come in contact with the innercylindrical wall of the sensor housing. At this temperature, the housingis expanded due to its high coefficient of thermal expansion. Becausethe glass is fluid there is substantially zero stress on the glass seal.

As the sensor is cooled, the glass seal 10 solidifies at temperature T₂(for example 750° C.), bonding to the temperature expanded sensorhousing or shell. As the sensor is continuously cooled, the sensorhousing contracts at a rate greater than that of the glass seal. Thiscauses the stress on the glass seal to fall below zero, maintaining theglass seal in compression between its upper and lower operatingtemperatures T₃ (for example 650° C.) and T₄ (for example -40° C.), withthe compressive stress increasing as the operating temperature of theglass seal region of the sensor decreases. By ensuring solidificationand bonding of the seal to the housing while the housing is above themaximum operating temperature, the seal is maintained in compressionover the entire operating temperature of the sensor.

EXAMPLE 4

FIG. 10 illustrates another example sensor according to this invention,similar to the example shown in FIG. 4 with the addition of chamfers 350and 352 on the top edge of the lower insulator 48. The chamfer 350extends entirely around the periphery of the top outer edge of insulator48 and the chamfer 352 extends entirely around the top inner edge of theinsulator 48 proximate to the sensing element 44. The sensor 40implemented with the chamfers 350 and 352 on insulator 48 exhibits goodperformance of the seal 10.

We claim:
 1. A sensor comprising:a tubular housing: a planar sensingelement axially within the tubular housing; and a glass seal between anexterior periphery of the planar sensing element and an interiorperiphery of the tubular housing, wherein the tubular housing has afirst coefficient of thermal expansion greater than a second coefficientof thermal expansion of the glass seal, wherein the second coefficientof thermal expansion of the glass seal is similar or equal to a thirdcoefficient of thermal expansion of the planar sensing element, whereinthe glass seal is maintained in radial compression over an entiretemperature operating range of the sensor and wherein cracks that formin the glass seal propagate to self limit and do not translate acrossthe glass seal to form leak paths.
 2. A sensor according to claim 1,wherein the glass seal comprises a circular disk portion and an annularcylindrical wall portion at an outer radial periphery of the flatcircular disk portion extending axially away from the flat circular diskportion in first and second directions opposite to each other, whereinthe annular cylindrical wall portion is in sealing contact with thetubular housing.
 3. A sensor according to claim 2, wherein the glassseal also comprises a rectangular opening in sealing engagement with theplanar sensing element.
 4. A sensor according to claim 1, also includingfirst and second cylindrical insulators on first and second axial sidesof the glass seal within the tubular housing, wherein the planar sensingelement passes through the first and second cylindrical insulators,wherein the first and second cylindrical insulators are in contact withthe glass seal, wherein at least a first portion of the firstcylindrical insulator in contact with the glass seal is in compressionand at least a second portion of the second cylindrical insulator incontact with the glass seal is in compression.
 5. A sensor according toclaim 1, wherein the glass seal has a circular disk portion with a firstthickness less than a second thickness of the planar sensing element.